An Avionics Technician reports he inadvertently left off fuselage access panel 191CB after trouble shooting and repairing a Left Wing Loop 'B' Duct Monitoring fault on a CRJ-900 aircraft.

Narrative

October 2011; I received a CRJ-900 aircraft with a Duct Monitoring fault; Left Wing Loop 'B' Open. While troubleshooting the problem; I removed the fuselage panel 191CB to gain access to the Duct Monitoring connectors MT168P2. The problem was very involved and required the removal of most of the leading edge wing panels as well as the fuselage panel. Upon fixing the problem by replacing the ceramic inserts in MT168P1 and MT170P1 [sensors]; I reinstalled all the wing panels. I inadvertently left off the fuselage panel 191CB. I was working by myself that night and had several more aircraft discrepancies to work on including two CAT II Checks. I am the only Avionics Technician here and most nights I am very busy. That night I was busier than usual with avionics work needing to be done on three different aircraft. When I did my Panel Check; I just walked the wing panels and due to the very limited lighting available (just my headlamp) I did not see the fuselage panel. I was very tired; as I had college classes the day before and had very little sleep with school and studying. I was not feeling well all night and was asked by my Supervisor if I was feeling alright; as I did not look well. I always do a Panel Check when I complete a job and I just completely missed the fuselage panel. [I was notified by] Maintenance Supervisor panel was replaced by robbing panel off a Heavy Check aircraft. Panel was recovered from ramp area and shipped to Heavy Check to replace it. Suggest better lighting on the Maintenance Ramp [and] more thorough Panel Checks.